Oregon monkeys with six parents push the limits of embryonic stem cells

The chimera of Greek mythology had a goat's body, a lion's head and a serpent's tail. Using cloning tools, Oregon researchers have created chimeras of a sort: monkeys grown from a mix of cells taken from as many as six monkey embryos.

It is a scientific first, and likely to prove useful for understanding human fertility, embryo development and the use of stem cells to treat diseases.

Animal cells from early-stage embryos can turn into any type of tissue in the body. Many researchers believe that one day human embryonic cells will repair damaged or failing organs – for example, regenerating heart muscle or severed spinal nerves. But to obtain the cells requires the destruction of human embryos, making the field ethically controversial.

"We are absolutely limited in our experimental use of human embryos," says University of Washington professor Carol Ware, who was not involved in the monkey study. "Any shred of information we can get from non-human primates is valuable."

The purpose of the work is to test lab-grown embryonic cells for their ability to diversify into all of the cell types and specialized tissues in the body. Forming a complete organism leaves no question about the potency.

"The chimera is the toughest but ultimate test of stem cells," says Mitalipov. "This gives you assurance that these cells are normal, functional, that they will replace diseased tissues."

Scientists created the first chimeric mice in the 1980s. They are now routinely used to study embryonic development and gene function. In mice, embryonic cells grown in a lab readily fuse with mouse embryos to form viable chimeric offspring. But the potency of embryonic cells from humans and non-human primates remains poorly understood.

In the experiments reported Thursday in the journal Cell, lab-grown embryonic cells of primates fell short. The researchers tested three lineages of lab-grown embryonic stem cells from rhesus monkeys. All failed to produce chimeric embryos. Only cells taken directly from embryos and fused together were capable of diversifying enough to produce living, baby monkeys.

"Most likely these stem cells we culture are not going to be that versatile in terms of making tissues and organs," Mitalipov says. "They are not as good as natural stem cells that reside in developing embryos."

Roku, Hex and Chimero started out as cells split from different embryos at the four-cell stage of development. Researchers combined cells taken from three to six embryos to create a single chimeric one. They placed 14 cell bundles into the wombs of five female monkeys. All five became pregnant, including two with singletons, two with twins, and one female carrying quadruplets.

The researchers aborted three pregnancies to examine seven fetuses. DNA tests confirmed that tissues throughout their bodies contained a random mixture of cells with non-matching genes reflecting their different parent sources.

Oregon researchers create cloned monkeys using variety of stem cellsDr. Shoukhrat Mitalipov of the Oregon National Primate Research Center speaks about experiments involving the use of different stem cells to create monkeys. Lab-cultured stem cells proved to be ineffective, while genuine stem cells taken directly from embryos resulted in the successful creation of monkey fetuses.

Two mothers carried pregnancies to term and gave birth to the twins Roku and Hex, and the singleton Chimero. All three look like typical males, but Roku proved to be made of a mixture of male and female cells. It's too early to know if Roku or the others will be fertile, Mitalipov says.

Ware, the UW professor, says she's not surprised that lab-grown stem cells failed to produce chimeric monkeys. Embryonic cells from human and nonhuman primates develop on a different schedule than those from mice. In lab culture, Ware says, the primate cells settle at a stage just beyond that of lab-grown mouse embryonic cells. When mouse cells are taken at that stage, they won't form chimeras, either, she says.

But she says Roku, Hex, and Chimero suggest it may be possible to select and grow embryonic cells in a lab that will be capable of fully diversifying into all of the body's tissues.

"Proof of principle is very often important to allow you to move forward," she says.

Mitalipov hopes the work will help researchers improve techniques for growing stem cells that keep their potency. He says the potency appears to be lost very early during isolation of cells from embryos, not during long-term culture, although that alters the cells to some extent.

"We have to go back and start working with embryonic stems cells that reside in an embryo and find out what makes them so potent," he says.

At least one animal rights group immediately condemned the work. The British Union for the Abolition of Vivisection (BUAV) called it "deeply disturbing" to subject animals as sentient as monkeys to such manipulations.

But political and financial support for the work has not been a problem. While the U.S. National Institutes of Health recently put a moratorium on new studies using chimpanzees, the policy does not apply to other primates. The Oregon researchers have six grants from the NIH to support their research.